1. Field of the Invention
The present invention generally relates to a magnet roller for use in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having at least two of these capabilities; and a developer bearer, a development device, a process cartridge, and an image forming apparatus including same.
2. Description of the Related Art
Electrophotographic image forming apparatuses typically include a latent image bearer, such as a drum-shaped or belt-shaped photoreceptor, on which electrostatic latent images are formed according to image data, and a development device to develop the electrostatic latent images. In electrophotographic image forming apparatuses, magnetic brush development methods using two-component developer consisting essentially of toner and magnetic carrier are widely used.
In magnetic brush development methods, developer is magnetically adsorbed onto an outer circumferential surface of the developer bearer, thus forming a magnetic brush. Then, in a development range formed between the developer bearer and the latent image bearer, toner is supplied from the magnetic brush to the electrostatic latent image formed on the latent image bearer, thereby developing it.
Developer bearers for use in such magnetic brush development methods typically include a cylindrical development sleeve constructed of a nonmagnetic material, and a magnet roller is provided disposed inside the development sleeve for generating magnetic force on the surface of the development sleeve. Magnetic carrier particles contained in developer are caused to stand on end on the development sleeve along the lines of the magnetic force thereon. Then, toner particles adhere to the magnetic carrier particles standing on end, forming a magnetic brush.
For example, JP-2001-165148-A proposes magnet rollers that include a cylindrical body and a pair of support portions provided on both sides of the body as shown in FIGS. 21 and 22.
A magnet roller 701 shown in FIG. 21 is shaft insertion type and includes a body 702 constructed of resin magnet and a metal shaft 705 penetrating the body 702 coaxially. Both ends of the metal shaft 705 serve as support portions 703 and 704.
The magnet roller 701 shown in FIG. 21 can have a high degree of rigidity and excel in durability owing to the metal shaft 705. However, if the magnet roller is reduced in diameter to respond to demands for compact image forming apparatuses, the volume of the resin magnet is reduced, resulting in insufficient magnetic force. Thus, it is difficult to reduce the size of shaft insertion type magnet rollers.
A magnet roller 801 shown in FIG. 22 is constructed of resin magnet, and a body 802 and a pair of support portions 803 and 804 are continuously formed by monolithic molding. Thus, the magnet roller 801 is monolithic molding type. Since the entire magnet roller 801 shown in FIG. 22 is constructed of resin magnet, this configuration can attain a stronger magnetic force than that attained by shaft insertion type magnet rollers.